1. Field of the Invention
The present invention relates to a laser device using an organic compound.
2. Description of the Related Art
Nowadays, laser light is used in various fields such as optical communication. Various types of laser light sources such as a solid-state laser, a semiconductor laser, a gas laser, a liquid laser (e.g., a dye laser), and the like are used as laser light sources. Among them, a semiconductor laser is small and lightweight, has high efficiency, and can operate at low voltage so that it is put into practice in many fields. However, emission wavelengths of a solid-state laser, a semiconductor laser, a gas laser, and the like, which use an inorganic compound as a medium, depend on the media. Therefore, the selection range of the emission wavelength is limited and it makes difficult to obtain laser light with a desired wavelength.
On the other hand, a dye laser uses a solution of an organic compound as a laser medium; therefore, the emission wavelength can be freely selected by adjusting a dye to be used and the shape of a resonator. However, the dye laser is poor in operability and is inconvenient since a large-scaled device to make a dye solution circulate is necessary in order to prevent degradation of the dye and a larger-scaled device is required to provide a light source in addition to the laser medium.
In order to solve this problem, it has been proposed to use not a solution but a thin film of a dye as a laser medium. The most direct solution is to provide a laser resonator structure inside an organic light-emitting diode (an organic EL element) to make a laser medium directly excite with current. In a laser light source having such a structure, an emission wavelength can be selected in a visible light range, and a large-scaled external excitation light source is not required. In addition, by sealing the organic light-emitting diode, entry of moisture and the like can be prevented, which prevents degradation of a dye thin film. Accordingly, it is easier to prevent degradation of the dye thin film compared to the case of using the dye solution. However, now, even when a high functional organic EL display is put into practical use, a current injection type organic laser device has not been put into practical use. There are two technical barriers of the current injection type organic laser device, which will be described below.
The first barrier is that the thickness of an organic thin film in an organic EL element is approximately from 100 nm to 500 nm. Since the distance between electrodes that are provided to sandwich the organic thin film is short, the optical gain of a resonator becomes low in the case where a resonator structure is formed in a direction perpendicular to a film surface. Further, light absorption of the electrodes is high so that the optical gain of the resonator becomes much lower. In addition, even in the case where a waveguide resonator structure in which the light propagation direction is parallel to the film surface is adopted, light absorption loss by the electrodes on both sides is still high.
The second barrier is that in a current injection type organic laser device, a light-emitting layer of an organic EL element is required to be a layer where laser light is emitted. In addition, it is also required to eliminate light absorption loss by all organic thin films included in the organic EL element. However, in an organic EL element manufactured using substances selected to meet such conditions, it is difficult to achieve all of high luminance, high efficiency, and high stability in operation.
As described above, it is difficult that a current injection type organic laser device in which a dye thin film is used as a laser medium and in which laser emission is performed by injecting current directly put into practical use. In such a situation, a method in which a dye thin film provided with an optical resonator is excited indirectly by a compact external light source is proposed (see Non-Patent Document 1 and Non-Patent Document 2). However, in any examples, a compact light source suited to the size of the dye thin film with an optical resonator has not been applied successfully.
In addition, there is a proposal which tried to complete an organic laser device by using an organic EL element as an excitation light source, which makes a light source compact (see Patent Document 1).